1. Field of the Invention
The invention relates to thermoplastic resin compositions and more particularly relates to improved polycarbonate and copolyester-carbonate resin molding compositions and articles molded therefrom and the method of improvement.
2. Brief Description of the Prior Art
Aromatic polycarbonate resins are a well known class of synthetic polymeric resins, generally prepared by the reaction of a polyhydric phenol with a carbonate precursor; see for example U.S. Pat. No. 3,989,672. Although such polycarbonate resins have been found to be thermoplastically moldable under a broad range of molding conditions, some polycarbonate resin compositions are not suitable for use in all molding operations because of a relatively high melt-flow behavior. Certain additives such as amine type aliphatic esters are known, which when admixed with many polycarbonate molding resins, lower the melt viscosity of the composition. However, the molded articles prepared from these compositions generally exhibit lowered impact strength because the polycarbonate is degraded by these additives.
We have discovered that olefin/vinyl carboxylate copolymers such as ethylene-vinyl acetate copolymers, when added to thermoplastic, moldable polycarbonate and copolyester-carbonate resins, are effective in reducing melt viscosities to a considerable degree, i.e.; by as much as two-thirds as measured by melt flow. This greatly improves processability of the resin in given molding operations. Unexpectedly, the improved processability is obtained without a significant loss of impact strength in molded articles prepared from the compositions of the invention. In fact, in some instances the impact strength may show improvement.
Ethylene-vinyl acetate copolymers have been used as additives in a minor proportion, to improve the stress crack resistance of polycarbonates to sterilizations conditions; see U.S. Pat. No. 4,145,373.